This invention relates to a noise detecting circuit adapted to be incorporated into an audio receiver or the like, and more particularly, to a noise detecting circuit that detects incoming of amplitude modulated waves that contain pulse-shaped noise signals and generates switching signals for controlling a gate circuit by breaking, for example, the main signal path between intermediate frequency transformers and eliminating the noise signals.
Generally speaking, upon receiving amplitude modulated waves, a change in the level of input electric field causes a far greater change in the level of audio output than upon receiving of frequency modulated waves. Therefore, even a relatively low level of pulse-shaped noise results in a low signal to noise (S/N) ratio depending upon the strength of electric field.
However, the conventional noise detecting circuit is based on the fact that pulse-shaped noise like that produced from a vehicle or car contains a high frequency component that lasts for a considerable long period of time, and produces a detector output higher than the average level of the desired signals removes any high frequency component from the detector amplitude of such high frequency, and when the signals reaches a certain level, the noise detecting circuit generates a control pulse signal that indicates detection of the noise. Accordingly, the conventional noise detecting circuit is only capable of detecting a relatively high level of pulse-shaped noise and is unable to detect a relatively low level of pulse-shaped noise that has, as mentioned above, substantially reduces the S/N ratio.
As for the noise eliminating circuit of FM stereo receiver, since noise contains a high frequency component, a high-pass filter is conventionally used to detect a component having a higher frequency than the desired FM frequency band, namely, higher than 75 KHz, a maximum modulated frequency of an FM stereo signal. The effect of eliminating noise can only be improved by ensuring detection of not only a high frequency component of noise but a low frequency component as well, but as mentioned above, the conventional noise detecting system has been unable to detect a low amplitude component of noise, although detection of such component is absolutely required for avoiding erroneous detection of a modulated signal frequency component.